1. Field of the Invention
The present invention relates to a fixing device that is used for fixing an unfixed toner image transferred onto a recording medium to form an image thereon, and an image forming apparatus that uses the fixing device.
2. Description of the Related Art
In general, an image forming apparatus using an electrophotographic process is equipped with a fixing device that heats and fixes an unfixed toner image transferred onto a recording medium (hereinafter also referred to as the “recording sheet”), and there are various types of heat sources used for the fixing device. In recent years, there is a demand for reduction of a warm-up time at the start of an image forming apparatus, and in view of this, there is a tendency to use a heater or the like which performs heating by so-called electromagnetic induction as a heat source.
A fixing device is generally provided with a fixing roller and a pressing roller, and a recording sheet is conveyed into a nip between the fixing roller and the pressing roller, where an unfixed toner image on the recording sheet is heated and fixed onto the recording sheet.
The above-mentioned fixing roller can be heated, and on-off control of the heater is performed based on a temperature detection signal obtained from a temperature sensor, such as a thermistor disposed on a surface of the fixing roller. Then, by the on-off control of the heater, the surface temperature of the fixing roller is controlled such that it becomes a predetermined temperature.
By the way, a recording sheet inevitably curls depending on the amount of toner attached thereto and the water content thereof. Then, due to the amount of curl, which is a degree of curling of a recording sheet, a recording sheet is sometimes wrapped around the fixing roller when fixing an image.
If a printing operation is performed in a state of the fixing roller in which a recording sheet is wrapped therearound, image defectivenesses, such as image roughness and faulty image fixing, may be caused on the image on the recording sheet when the printing operation has been executed, since the surface of the fixing roller is covered with the recording sheet.
Further, in a heater using electromagnetic induction heating, if a Curie material is used for the fixing roller, the surface temperature of the fixing roller is detected on the recording sheet wrapped around the fixing roller. That is, the surface temperature of the fixing roller is detected via the recording sheet, and as a result, the actual surface temperature of the fixing roller becomes higher than the temperature detected by the temperature sensor. Further, the surface temperature of the fixing roller sometimes becomes higher than the Curie temperature.
At this time, the impedance of an electromagnetic induction heating coil is rapidly reduced, and hence overcurrent flows into the electromagnetic induction coil, which sometimes causes failures, such as blowout of an AC fuse of the power source.
As mentioned above, when a failure occurs in the power source, it takes much time to recover the power source, which increases downtime in which the image forming apparatus is not available. To prevent such an inconvenience, it is necessary to immediately detect that a recording sheet is wrapped around the fixing roller. In short, it is required to reduce time to detect a wrap jam of a recording sheet.
To this end, there has been proposed a fixing device that detects a wrap jam of a recording sheet by disposing recording sheet-detecting sensors for detecting a recording sheet, on an inlet side and an exit side of the fixing device, respectively (see e.g. Japanese Laid-Open Patent Publication No. 2004-354983).
In Japanese Laid-Open Patent Publication No. 2004-354983, it is determined whether or not a recording sheet is wrapped around the fixing roller based on results of detection by the recording sheet-detecting sensors, and depending on a result of the determination, the driving and heating of the fixing roller are stopped.
Further, there has been also proposed a fixing device of a belt type in which a noncontact temperature sensor is disposed in an area where a sheet passes and a contact temperature sensor is disposed in an area where no sheet passes (see e.g. Japanese Laid-Open Patent Publication No. 2006-251488).
In Japanese Laid-Open Patent Publication No. 2006-251488, when the temperature detected by the contact temperature sensor becomes equal to a predetermined temperature, whether or not a recording sheet exists between the noncontact temperature sensor and a fixing belt is judged depending on whether or not the temperature detected by the noncontact temperature sensor has reached a predetermined temperature.
By the way, in the image forming apparatus described in Japanese Laid-Open Patent Publication No. 2004-354983, when a paper jam is caused, the user takes the fixing device out of the body of the image forming apparatus to perform jam removal. In this jam removal, the user manually rotates a fixing roller, which sometimes causes a recording sheet to be wrapped around the fixing roller.
If the power of the image forming apparatus is restored in this state, the recording sheet cannot be detected by the recording sheet-detecting sensor disposed on the inlet side of the fixing device, so that the fixing roller is heated.
To prevent such a situation, it is necessary to restrict manual rotation of the fixing roller by a user in jam removal by adding e.g. a mechanism for restricting the direction of rotating the fixing roller to thereby prevent a recording sheet from being wrapped around the fixing roller. However, there is a problem that addition of such a mechanism inevitably causes costs to be increased.
On the other hand, in the image forming apparatus described in Japanese Laid-Open Patent Publication No. 2006-251488, when the temperature detected in the area outside the sheet-passing area has reached a predetermined temperature, it is judged whether or not the temperature detected in the sheet-passing area is correct. Thus, the judgment on whether or not the temperature detected in the sheet-passing area is correct is performed according to the temperature detected in the area outside the sheet-passing area, which depends largely on the environment temperature, and hence it is difficult to uniquely determine a set value (i.e. predetermined value) for the temperature detected in the sheet passing area depending on the environment where the image forming apparatus is installed, or the last printing operation.
Further, if the contact temperature sensor disposed in the area outside the sheet-passing area is not normally brought into abutment with the fixing belt, it is becomes impossible to detect an increase in the temperature of an end portion of the fixing belt during the printing operation, and hence even when the difference between the actual temperature in the area outside the sheet-passing area and the temperature in the central area of the fixing belt is large, it is sometimes judged that the state of the fixing device is normal.
Therefore, in the image forming apparatus described in Japanese Laid-Open Patent Publication No. 2006-251488, there can be an increase in the difference between temperatures in the central area and the end portion of the fixing belt, causing an inconvenience, such as generation of wrinkles on a recording sheet.